How to combine two middle-grey images and keep the same value?
To combine two textures with a middle-grey and keep the same value connect your previous image to an overlay node and the new in the second slot.

Because Blender uses sRGB and not a Linear color management you need to correct the gamma by connecting it to a gamma node in the image slot and for the second slot connect a math Divide node with 1 on top et 2.2.